1. Field of the Invention
This invention relates to a method and apparatus for thermal transfer printing for thermal transfer of an object, such as a picture pattern or a letter formed on a thermal transfer film, onto a printing surface of a support of metal or ceramics having high heat dissipating characteristics.
2. Description of Related Art
A disc-shaped recording medium, such as a floppy disc, a magnetic disc or a magneto-optical disc, is rotatably accommodated in a cartridge main body, combined from an upper cartridge half 2 and a lower cartridge half 3, to constitute a disc cartridge, as shown for example in FIG. 1. By enclosing a disc-shaped recording medium 4 in the cartridge main body in a sealed manner, the surface of the disc-shaped recording medium 4 may be protected against possible grazing or deposition of foreign matter, such as dust and dirt.
The cartridge main body has an information signal recording/reproducing aperture for exposing a part of a signal region of the disc-shaped recording medium 4 to outside across the inner and outer rims of the signal region of the recording medium 4. In order to prevent dust and dirt from entering and becoming deposited on the surface of the disc-shaped recording medium 4, the information signal recording/reproducing aperture is closed, during the time the disc cartridge is not in use, by a shutter member 5 movable along a lateral side of the cartridge main body 1. The shutter member 5 is formed by punching and bending a thin metal plate and is comprised of a pair of shutter sections for opening and closing the information signal recording/reproducing aperture, and a connecting web section interconnecting proximal sides of the shutter sections.
The shutter member 5 has the name of the producer, kind or type of the article and the model name 6, printed on the surface of one of the shutter sections. The names of the producer, kind or type of the article or the model name 6, mainly provided from a functional aspect, have hitherto been formed by a direct printing method, such as a silk screen printing method or a pad printing method. Recently, disc cartridges are processed decoratively for distinction from peer products. That is, the name of the producer, kind of the article and the model name 6 are printed on the shutter member 5 in multiple colors, while a picture pattern 7 indicating the contents of the information signals recorded on the disc-shaped recording medium 4 is also printed on the shutter member 5.
On the other hand, industrial ceramics, excellent in heat resistance or resistance against impact or abrasion, are employed in products of various sectors. These ceramic products are also frequently supplied as components and the name of the producer, kind of the article or the model number is printed by the direct printing method, such as a silk screen printing or pad printing, from functional aspects.
Meanwhile, the direct printing method, such as the silk printing method or the pad printing method, employed for printing on a metal or ceramic product, such as the above-mentioned shutter member for the disc cartridge, necessitates a large number of plates corresponding to the number of colors which are to be in use, so that printing accuracy is lowered due to shifting of the plate position, resulting in an increased number of printing steps and cost. Consequently, the number of colors employed in multicolor printing on metal or ceramic products cannot practically be increased to more than two at most. With such a small number of colors, decorative printing, which distinguishes the product from peer products, cannot be expected.
In order to solve the above-mentioned problem inherent in the direct printing method, it is contemplated to employ a thermal transfer printing method employing a thermal transfer film 10, as shown for example in FIG. 2. A thermal transfer film 10 comprises a base film 11, formed of, for example, polyester or polyolefin, and having a transparent release layer (protective layer) 12 of vinyl chloride or a vinyl acetate copolymer formed thereon. A printing layer 13, carrying a letter or a picture pattern thereon, is formed by printing on the release layer 12, and a heat-sensitive adhesive layer 14 is formed for sheathing the printing layer 13.
Consequently, if, after applying the thermal transfer film 10 on the printing surface of a support (e.g., shutter member 5), the thermal transfer film 10 is pressed onto the support by a thermal transfer head, the heat-sensitive adhesive layer 14 is melted so that the printing layer 13 and the release layer 12 are transferred onto the support. The letter or the picture pattern, formed on the thermal transfer film 10, is printed by releasing the base film 11 and leaving the printing layer 13 and adhesive layer 14 on the printing surface. This process is shown sequentially in FIGS. 2 to 5.
The thermal transfer printing method, employing the thermal transfer film 10, enables multicolor letters or picture patterns to be printed on the support in a simpler manner, and hence may be advantageously applied to products or components which are in need of decorative processing and must be produced at low cost.
Although the thermal transfer printing is highly effective for multicolor printing, heat dissipation from the thermal transfer head occurs from the ceramic product or the metal product having high heat radiation characteristics, with the result that the melting temperature of the heat-sensitive adhesive layer 14 of the thermal transfer film 10 is hardly reached. As a result, the letter or the picture pattern formed on the thermal transfer film 10 is partially left on the thermal transfer film 10 without being transferred in an optimum state to the support, such as the metal product or the ceramic product. On the other hand, the heating time for heating the thermal transfer film 10 is protracted, thereby lowering the printing efficiency.
For overcoming these problems, it may be contemplated to set the temperature of the thermal transfer head to a higher temperature and to press the thermal transfer film 10 at a higher temperature. However, if the thermal transfer film 10 is heated to too high a temperature, the printing layer 13 formed thereon tends to be destroyed. Consequently, the thermal transfer printing method, which is universally employed for products having low heat radiating (i.e., heat dissipating) characteristics, such as products of synthetic resin or porcelain, cannot be applied without considerable difficulties to the printing of letters or picture patterns on the surface of metal or ceramic products having high heat radiation characteristics. Therefore, there has been little application of multicolor printing on these products.